The invention concerns a process for manufacturing alkenyl dicarboxylic acid anhydrides.
The alkenyl dicarboxylic compounds, acids or anhydrides, are important starting materials for manufacturing additives for fuels and lubricants. The products usually employed consist of a hydrocarbon chain with a mass of from 200 to 2000, on which is attached at least one succinic group.
Manufacturing processes starting from an olefin oligomer and maleic anhydride are known in the prior art. The condensation is performed at a temperature higher than 200.degree. C. and the reaction time must be sufficiently long to obtain a conversion of about 75%. Processes of this type are described, particularly in the U.S. Pat. Nos. 2,628,942 and 2,634,256.
To improve the conversion, it has been proposed, in French Pat. No. 2,257,604, to use maleic anhydride in excess; however, even in that case, the condensation temperature remains higher than 200.degree. C., the reaction time is about 20 hours and the process necessitates the step of removing excess maleic anhydride at the end of the reaction.
It has thereafter been proposed to prepare compounds of the same nature by condensing maleic anhydride with a previously chlorinated polyolefin containing approximately one chlorine atom per molecule of the polyolefin. This technique is described, for example, in the British Pat. No. 922,831, corresponding to U.S. Pat. No. 3,172,892. The reaction time is shorter and the operative temperature is not as high as when condensing the polyolefin itself. The secondary reactions which result in the formation of polymeric resins are also avoided. It is also mentioned that the unsaturated anhydrides decompose from 150.degree. C. upward.
Attempts have been made to simplify this process, particularly by performing the chlorination and the condensation in one step: for example, the British Pat. No. 949,981 discloses the chlorination of a mixture of polyisobutane with maleic anhydride at a temperature of 190.degree. to 210.degree. C., the condensation time being, under these conditions, 10 h.
The British Pat. No. 1 356 802 describes the addition of iodine to the reaction mixture after a 25% conversion; however, under these conditions, the reaction temperature is 215.degree. C. and the reaction must be continued up to 24 hours to obtain satisfactory conversions.
The French Pat. No. 2 273 014 shows that bromine can be used to catalyze the reaction, but the reaction temperature is 240.degree. C. in the described conditions.
In a number of patents, the halogens used to catalyze the reaction of the polyolefins with maleic anhydride are not used in the free state but combined with other organic molecules; for example, the U.S. Pat. No. 3,927,041 describes the use of a 1,3-dibromo-5,5-dialkyl hydandoin and the U.S. Pat. No. 3,954,812 shows that acid chlorides, chloro-acids and N-bromosuccinimide may be used as halogen generators.
The mechanism by which the halogen compounds operate has not been perfectly explained, but it is probable that the polyolefin halogenates and then dehydrohalogenates prior to the condensation with the anhydride: this path is described in the U.S. Pat. No. 3,864,270 where is is shown that the polyolefin can be chlorinated at between 20.degree. and 150.degree. C., then dehydrochlorinated at between 150.degree. and 250.degree. C., preferably at 235.degree. C. for 5 hours. After these operations, maleic anhydride is condensed for 6 hours at about 100.degree. C.
It is clear that the dehydrochlorination step is the step which necessitates the higher temperature.